CHAPTER I. 

 THE FIELD OF VISION. 



In studying the modifications met with in the visual 

 organs in the various species of Mammalia, to allow for 

 different requirements in connection with the visual field, 

 we have to take into consideration the monocular field, the 

 combined monocular fields, the binocular field and the 

 field of fixation. 



The extent of the monocular field may be modified by 

 the degree of prominence of the eye in the head, and by 

 structural alterations in the eye itself. 



In animals in whom a large monocular field is of the greatest 

 importance, such as those who depend for safety on their 

 rapidity of flight, the eyes are found set prominently out 

 from the surface of the head. In the Ungulata the outer 

 margin of the orbit is composed of a complete projecting 

 bony ring which serves to hold the eyeball prominently 

 forward (Fig. 1). In the Rodentia, the Insectivora and 

 the Carnivora the orbital ring is incomplete externally, in 

 the latter the malar and frontal apophysis being only united 

 by fibrous bands (Fig. 2). 



The evolution from an incomplete to a complete bony 

 ring at the margin of the orbit can be well traced in tin- 

 ancestry of the horse. In the Meso-hippus baridi, its three- 

 toed ancestor, from the lower Miocene tertiary of North 

 America, the bony wall of the orbit is open behind. 



In monkeys, anthropoid apes and in man the bony 



